<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .l, .c, .r {
            height: 600px;
            float: left;
        }
        .l {
            width: 400px;
            background-color: aqua;
            /* 为了让l元素从当前行移动到第一行同一位置*/
            margin-left: -100%;
        }
        .c {
            width: 100%;
            background-color: blue;
        }
        .i {
            height: 600px;
            background-color: blueviolet;
            margin-left: 400px;
            margin-right: 400px;
        }
        .r {
            width: 400px;
            background-color: brown;
         /* 为了让r元素移动到第一行*/
        margin-left: -400px;
    }
</style>
</head>
<body>
    
    <div class="c">
        <div class="i">自适应</div>
    </div>
    <div class="l">定宽</div>
    <div class="r">定宽</div>
</body>
</html>